what causes my 1999 pontiac firebird to sputter while idling and while driving

When was the last time you did a tuneup on your bird?

You might have to do a throttle body cleaning and EGR valve cleaning. Theres a throttle body air sensor MAF mass air flow that could be dirty it could be metering in the wrong air flow and the computer will get the wrong readings and it might cause a car to do what your car is doing so you have to buy a special cleaner for that you can't just spay it with any cleaner and never touch it or wipe it. You can also remove the throttle body from the car and clean inside of it with a brush and if it has a MAF sensor that's the part you're not supposed to touch "mentioned before" so read carefully what you're supposed to do on the can of the cleaner and get some more insite before you attempt to do this or have somebody professionally do it that's all I can think it could be? PS if you remove the throttle body and or the EGR valve you should buy replacement gaskets for both of those. it's three things to make certain that you have a thorough tune up and its to clean EGR valve clean the throttle body and clean the MAF sensor so those are area's s it could be making your car do what it's doing good luck

if you didn't want to remove anything you could just clean it the best you can you know by opening up the plumbing part of the air hose that goes to your throttle body and then shoot some cleaner inside of the throttle body so you don't have to remove the throttle body you can just take the air hose going to it remove it get it out of the way and then spray cleaner inside of the throttle body but to do a thorough job you should take it off and clean the inside of it. The EGR valve you could probably just you know take it off and clean it as best you can say that's what I would do and then put it back on.

Have you changed your fuel filter? I would start there since you mentioned the sputtering gets worse at high way speeds. Also, there is a MAP sensor AND a MAF sensor on the series II 3800. Have you checked the codes with a scan tool? Most part stores will check your codes for free. I recently bought a cheap scanner for $20 at Manards. My '99 Firebird has many electrical issues and Im slowly getting them figured out with the help of the scanner. If, upon inspection, your MAF and MAP sensors appear clean, check the pigtails and clean and grease the contacts in the plugs before snapping them back on the sensors, and check for frayed wires. (dielectric grease and electronic parts cleaner, both available at the parts store)make sure the contacts are completely dry before greasing and snapping them back on. If there isn't much improvement in drivability after doing all this, rent a fuel pressure tester and make sure your fuel pump is healthy, (dont forget to change the filter first)